Forced air heating systems utilizing gas or oil fired burners as a means of heating the heating media (air) are usually comprised of the following components:
a) Thermostat; which senses the temperature within the desired space and activates the furnace's burner. PA1 b) Burner; which generates a flame and hot-gases. PA1 c) Heat-exchanger; the device used to transfer the flame and hot-gas temperatures to the heating media (air). PA1 d) Heating media distribution means; usually ductwork. PA1 e) Circulating Fan; used to force the heating media through the distribution means. PA1 f) Items within the controlled environment having thermal mass and inertia. PA1 a) The worst case scenarios (design-loads) that the systems are expected to encounter. PA1 b) Square footage and other architectural considerations of the facility. PA1 c) Anticipated future expansions. PA1 d) Expected degradation of the system output due to aging.
A typical residential forced air heating system is usually controlled in the following manner:
When there is a need for heat within the space, the space thermostat calls for heat directly energizing the burner. Once a certain temperature is reached within the furnace's heat exchanger, the air-circulation fan is started independently using it's own built-in thermostat. The air-circulation fan forces the heating media through the distribution means and causes a heat increase within the controlled space. When the desired space temperature setpoint is reached, the space thermostat de-energizes the burner. The air-circulation fan continues to run until the temperature within the heat exchanger drops to a certain temperature (as set via the units built-in heat exchanger thermostat). The above control scheme is repeated over and over again as a means of controlling the space temperature. In a typical commercial (roof-top furnace) application, the air-circulation fan may run continuously.
In connection with heating systems, it is common knowledge that the output capacities of heating systems are usually determined by:
Anytime the demand on the heating system is less than the heating capacity of the system, the heating system is over-sized. This over-sizing condition exists, within a typical properly designed system, about 85% of the time and causes the heating system to cycle the burner as the means of controlling the temperatures within the desired space.
Experimentation has shown that the temperature of the air being discharged from the furnace has a terminal (maximum) temperature that is reached, regardless of how long the burner is firing for. This terminal temperature is reached whenever the furnace is being utilized at less than maximum design load and is caused by the inability of the heat exchanger to transfer the total heat generated by the flame and hot gases to the heating media. This inability of the heat exchanger is partially due to inefficiencies of the heat exchanger itself, and partially due to the inability of the heating media to absorb all of the heat that the burner is capable of generating. Keeping the burner firing during this terminal temperature period is not productive and wasteful because the heat that is not absorbed by the heating media is expelled as hot gases, usually through the flue system.
Experimentation has also proven that additional thermal energy is available in the heat exchanger itself. This energy can be utilized during the relatively brief off period of the burner (generated by the invention) to maintain adequate heating discharge air temperatures.
Fuel savings are achieved, while maintaining the same space temperature conditions, by intelligently cycling the burner about this "terminal temperature", and by utilizing the additional heat available for extraction from the heat exchanger.
The thermal inertia and thermal storage of the items within the controlled space are used as a capacitor, of sorts, to absorb any short-term thermal transitions.
It has also been shown experimentally that while cycling the burner about the terminal temperature of the heat exchanger does lead to fuel reduction, it is necessary for the invention to not allow the discharge air temperature to drop too low. Too low, is the point at which there would be insufficient heat energy available to provide heating for the space and/or the point at which the air circulator fan would undesirably stop during a heating call. The invention described herein will not allow this to happen by cause of the invention.